The present invention relates to a method for assembling an operating rig for a fluid in a body of water, of the type comprising the following steps:                bringing a buoy to the surface of the body of water substantially facing an anchoring region on the bottom of the body of water;        connecting a downstream point of a fluid transport pipe to the buoy;        completely immerging the buoy under the surface of the body of water before or after the connection step;        deploying in the body of water an intermediate section of the transport pipe from the downstream point to at least as far as an upstream point;        anchoring the upstream point on an anchoring element attached to the bottom of the body of water in the anchoring region;        tensioning the intermediate section of the transport pipe between the downstream point and the upstream point under the effect of the buoyancy of the buoy, in order to maintain the intermediate section substantially vertical in the body of water;        connecting onto the intermediate section a lower section intended to be connected to the bottom assembly and an upper flexible section intended to be connected to the surface assembly.        
Such rigs are intended for transporting a fluid produced in the bottom of a body of water up to the surface through the body of water. This fluid notably consists of liquid and/or gas hydrocarbons and of water collected in production wells made in the bottom of the body of water.
Such a rig generally comprises a lower pipe for connecting to the production assembly positioned on the bottom of the body of water, a substantially vertical rising column, made on the basis of a flexible pipe or a rigid tube, a buoy for maintaining under tension the rising column in its vertical position, and an anchoring element of a lower point of the rising column.
The rig further comprises an upper flexible connecting pipe connecting the rising column to a floating surface assembly.
Thus, the hydrocarbons produced by the bottom assembly are successively transported through the lower connecting pipe, the rising column and the upper connecting pipe up to a surface assembly such as a ship, a platform or a barge, where they may be recovered or transported.
This type of rig has a relatively simple structure, since maintaining it in a vertical position is exclusively ensured by the anchoring element in the bottom of the body of water, and by the tension generated by the buoyancy of the maintaining buoy connected to the upper point of the rising column.
However, such rigs remain difficult to install, notably because of the depth of the body of water, as well as movements at the surface of the body of water due to swell and/or wind.
Thus, the lifting buoy, after having been transported on a ship as far as the site for installing the rig, has to be immersed at a sufficient depth so as not to experience the effects of the swell and of the current. For this purpose, the buoy is gradually lowered in the body of water by lifting it out of the ship with a handling crane as for example described in FR 2 911 907.
In order to allow the pipe to be held vertically in the body of water, the buoy has to be very voluminous, taking into account the weight of the rising column. Thus, the buoy may have a diameter of more than several meters for a height of several tens of meters.
The buoy generally has an elongated cylindrical shape along a vertical axis, notably for facilitating connection of the rising column to the upper connecting pipe, when this connection is carried out under the buoy.
Such an installation method therefore requires the availability of a ship having a lifting crane with very large capacity and a great height.
Such a ship is not always available. Further, the vertically elongated buoys are very unstable upon their immersion in the body of water.